The exposure of critical equipment to salt water and the resulting corrosion is a large and costly issue. To combat corrosion, surfaces have traditionally had cadmium coatings applied to provide sacrificial galvanic protection against corrosion. Currently, cadmium coatings for critical components such as landing gear, are often vacuum deposited through a thermal evaporation physical vapor deposition (PVD) process to protect the surfaces from the corrosion environment. Unfortunately, cadmium is a hazardous substance, a known human carcinogen and extremely expensive to dispose. Thus, Executive Order 13423, Occupational Safety and Health Administration (OSHA) guidelines, and the DOD “Listing of Toxic Chemicals, Hazardous Substances, and Ozone Depleting Chemicals (Defense Standardization Program SD-14)” all require the federal government to reduce usage of this material.
Cadmium coatings provide corrosion protection for steel based components through the slightly lower electrochemical potential of cadmium, providing excellent galvanic protection to the underlying steel. This low potential difference between cadmium and high strength steel is below the critical potential for stress corrosion cracking and also does not overlap the potential range for hydrogen cracking. Replacement coatings for Cd would be required to have an electrochemical potential that is in this range. In addition to the corrosion requirements, new coating compositions would also be desired to display strong adherence to the substrate, hydrogen embrittlement resistance, no fatigue debit to the substrate, and provide impact resistance.
The constant exposure of Navy equipment to salt water insures that corrosion is a large and costly issue, estimated to cost roughly $20 billion every year. Traditionally vacuum plated cadmium is used to combat corrosion due to: the excellent galvanic protection provided to steel substrates; the good coating density; and the limited effect on the underlying substrate strength.
In general, aluminum and zinc coatings have the corrosion resistance and sacrificial properties required for many cadmium replacement applications. For high strength steel, tailhook components in particular, improved corrosion resistance over that of pure Al coatings is required. Desired properties include: slightly reduced electrochemical potential of the coating relative to the steel substrate; strong adhesion to the underlying substrate; long life of corrosion protection; and limited substrate fatigue debit caused by the coating.
It is therefore desired to incorporate alloying elements into aluminum coatings to generate coatings with the electrochemical potentials closer to that of the steel substrate. However, currently no viable deposition process exists to enable the use of aluminum alloy coatings for cadmium replacement.
To develop coating compositions and processing approaches which are environmentally friendly, but still retain the corrosion protection achievable traditionally with Cd coatings, several key issues must be kept in mind to generate successful coatings. These include: coating must be effective at protecting the substrate from corrosion and embrittlement; dense coatings to prevent attack of the substrate; able to be deposited into non line-of-sight regions of a component (e.g. the tailhook component), retain the strength of the underlying substrate after coating; and non-toxic materials for the coating composition and as precursors for processing.
Aluminum is a good material for corrosion resistant coatings due to the closeness of the electrochemical potential to steel and the properties of the aluminum oxide layer that forms on the surface upon oxygen exposure. The aluminum oxide scale is thin, slow growing (leading to enhanced stability), has good adhesion to underlying layers and forms a protective layer on the surface. This layer of aluminum oxide can be reformed if the oxide scale is removed, thus a number of existing methods have been investigated for depositing aluminum corrosion resistance coatings, each have corresponding shortcomings. Perhaps the most widely used replacement technology for cadmium corrosion resistant coatings is ion vapor deposition (IVD) of aluminum. IVD is an environmentally clean, commercially available processing approach based on physical vapor deposition (PVD), but is limited by poor coating density, long chamber pump down times and an inability to coat regions not in the line-of-sight of the vapor source. The inability to coat non line-of-sight (NLOS) areas is particularly troublesome for complex geometries such as the landing gear, and hollow fasteners with attachment holes, blind holes, concave and convex shape features. In addition, aluminum coatings deposited using the IVD technique are used in stringent corrosive environments only after additional treatments to increase the density of the coating and the application of easily damageable sacrificial topcoats. Often the modifications are through the use of chromate conversion which introduce toxic chemicals into the processing of these coatings, and are desired to be removed.
Other approaches currently being suggested to deposit corrosion resistant coatings also have limitations. For example, electroplating of aluminum such as Alumiplate has exhibited good corrosion protection, and electroplating can deposit even coatings over complex geometries, however, the electroplating of aluminum alloys is difficult since the chemistry is dependent on the continuously changing solution chemistry and the current density. Aluminum electroplating processes also require the use of toxic chemicals which can limit their application. The incorporation of hydrogen into the coating during the electroplating process can also lead to hydrogen embrittlement of the substrate unless post deposition heat treatments are applied. These additional processing steps add time and cost to the coating application process and hazardous waste continues to be an issue even with the non-Cd chemistries.
Chemical vapor deposition (CVD) and thermal spray are also under consideration for niche applications in corrosion coating deposition, but these also have many technical limitations. In CVD, coatings can be deposited onto interior surfaces because the flux is distributed across the part surface using a gas flow. However, the deposition process requires the use of toxic (and frequently expensive) precursor materials. In addition, the deposition of the multi-component metallic alloys is difficult, deposition rates are low, and high substrate temperatures are often required. Thermal spray coatings of Al and Al—Zn are used on some aircraft components as these approaches are commercially available and can coat large areas economically. However, the coatings are thick, rough and often porous limiting their effectiveness for many components, such as landing gear and fasteners.
Pure aluminum coatings have been shown to be a suitable Cd replacement in some applications, although increased performance i.e. increased protection time and less fatigue debit may be obtained through the incorporation of alloying elements to tailor the electrochemical potential closer to that of the steel substrate.
Physical vapor deposition processes such as sputtering and electron beam-physical vapor deposition (EB-PVD) typically deposit the highest quality coatings enabling thin, dense layers (under some conditions) of a wide range of coating materials. For these cases, dense layers are only obtained if depositing atoms have sufficient surface mobility to diffuse to low energy sites on the deposition surface. This can occur via thermal energy (i.e. from the heat of adsorption or from heating the substrate) or via kinetic energy (i.e. high velocity atom impacts resulting from plasma activated processes). PVD approaches are growing in interest in many applications because they do not introduce additional harmful chemicals. These processes, however, often operate under a stringent vacuum condition that increases their ultimate cost (primarily due to difficulties in achieving and maintaining ultra high vacuum) and limits their deposition to regions in the line-of-sight of the vapor source.
However, currently no viable deposition process exists to enable the use of aluminum alloys for cadmium replacement.